The present invention relates to a brake system that has a brake assist feature implemented in brake circuits and activated during sudden or panic braking.
For example, Japanese Unexamined Patent Publication No. 11-108230 discloses such a brake system. This brake system has a couple of brake circuits, one for a pair of vehicle wheels and one for another pair of vehicle wheels. The brake assist feature is implemented in the brake circuits by a couple of pumps (one for each brake circuit) and a couple of differential pressure retaining valves (one for each brake circuit).
In the above-described brake system, if a discharge capacity differs between the pumps, or if differential pressure retaining ability differs between the differential pressure retaining valves, a second brake fluid pressure, which is generated in each brake circuit and is higher than the master cylinder pressure, may differ between the brake circuits. For instance, the difference in the second brake fluid pressure can be created when a difference in resistance of electric lines provided for supplying electric power to each pump or differential pressure retaining valve exists between the brake circuits for some reasons. Furthermore, the difference in the second brake fluid pressure can also be created when a difference in pressure-sealing performance of the pump exists between the brake circuits due to, for example, aging.
The creation of the differential pressure between the brake circuits causes some problems. For instance, in a diagonally split brake system having one brake circuit for the right front wheel and the left rear wheel and one brake circuit for the left front wheel and the right rear wheel, it causes a difference in braking force between a left side and a right side of the vehicle. Furthermore, in a vertically split brake system having one brake circuit for the right front wheel and the left front wheel and one brake circuit for the right rear wheel and the left rear wheel, it causes a difference in braking force between a front side and a rear side of the vehicle.
The difference (unbalance) in the braking force can be particularly large in the vehicles having the brake assist feature. That is, during panic braking, during malfunction of a brake booster or during the operation beyond a dead point of the brake booster, when the wheel cylinder pressure greater than the master cylinder pressure is generated in the brake lines and is supplied to each wheel, the wheel cylinder pressure supplied to each wheel may have no relationship or a relatively smaller degree of relationship with the master cylinder pressure. Thus, a difference in brake fluid pressure between the wheels or between the brake circuits cannot be substantially compensated. Furthermore, since the high brake fluid pressure supplied to each wheel is greater than the master cylinder pressure, the difference in the brake fluid pressure that causes the above-described difference in the braking force may be increased.
The present invention addresses the above-described disadvantages. Therefore, it is an objective of the present invention to provide a brake system having a brake assist feature implemented in brake circuits and being capable of compensating a difference in brake fluid pressure between the brake circuits and thereby keeping the difference in the brake fluid pressure between the brake circuits to be less than or equal to a predetermined amount to assure a sufficient stability of vehicle motion during braking aided by the brake assist. In a case of vertically split brake system, the difference in the brake fluid pressure between a front wheel brake circuit and a rear wheel brake circuit should remain less than a predetermined amount to maintain a predetermined brake force allocation between the front wheels and the rear wheels.
To achieve the objective of the present invention, there is provided a brake system for a vehicle having a master cylinder, first and second wheel cylinders, a first brake circuit, third and fourth wheel cylinders, a second brake circuit, first brake assist means and second brake assist means. The master cylinder has first and second chambers in each of which a master cylinder pressure is generated in response to brake operation by a vehicle driver. The first and second wheel cylinders are provided for exerting a first braking force on first and second vehicle wheels, respectively, by a first wheel cylinder pressure generated based on the master cylinder pressure in the first chamber of the master cylinder. The first brake circuit connects the master cylinder to the first and second wheel cylinders. The third and fourth wheel cylinders are provided for exerting a second braking force on third and fourth vehicle wheels, respectively, by a second wheel cylinder pressure generated based on the master cylinder pressure in the second chamber of the master cylinder. The second brake circuit is provided for connecting the master cylinder to the third and fourth wheel cylinders. The first brake assist means is arranged in the first brake circuit to increase the first wheel cylinder pressure to a level higher than the master cylinder pressure in the first chamber and to conduct the increased first wheel cylinder pressure to at least one of the first and second wheel cylinders in response to at least one of a state of the brake operation by the vehicle driver and a braking state of the vehicle. The second brake assist means is arranged in the second brake circuit to increase the second wheel cylinder pressure to a level higher than the master cylinder pressure in the second chamber and to conduct the increased second wheel cylinder pressure to at least one of the third and fourth wheel cylinders in response to at least one of the state of the brake operation by the vehicle driver and the braking state of the vehicle. The second brake assist means includes a brake regulator mechanism that uses the increased first wheel cylinder pressure as a pilot pressure and adjusts the increased second wheel cylinder pressure in the third and fourth wheel cylinders to a pressure falling within a predetermined range from the increased first wheel cylinder pressure in the first and second wheel cylinders when the first brake assist means is activated.